WC-Co　cemented　carbides　were　prepared　by　sintering　thein-situ　synthesized　composite　powders　with　a　constant　Co　content　and　VC　or　Cr3C2particles　of　different　size　scales.　With　an　optimized　carbon　addition　and　doping　of　the　microscale　VC　or　nanoscale　Cr3C2particles,　the　ultrahigh　fracture　strength　with　mean　valuesabove　5000　MPa　have　been　obtained　in　the　prepared　cemented　carbide　bulk　materials.　By　detailed　crystallographical　analyses　of　the　configuration　and　interactions　of　WC,　Co,　VC　and　Cr3C2　phases,　the　effects　of　the　VC　and　Cr3C2particle　size　on　the　microstructure　and　mechanical　properties　of　the　cemented　carbides　were　demonstrated.　The　mechanisms　for　the　reinforcement　of　the　fracture　strength,　concerning　the　binder　distribution,　characteristics　of　phase　boundaries　and　dislocations　motion　in　the　WC　grains　were　proposed,　and　the　dominant　factors　for　the　ultrahigh　fracture　strength　of　the　cemented　carbides　were　discussed.